Having a large number of a particular tree species in an area is not necessarily good by default – there must also be good genetic diversity of the population. Whilst there have perhaps not been too many total extinctions as a result of poor genetic diversity of a tree species, a lack of diversity does mean that a tree species is very susceptible to a pest or pathogen. We can look at the case of Castanea dentata to prove this.
The American chestnut, with a native range from New England to Alabama, would have accounted for up to 40% of the total forest overstorey cover, supported a wide range of species, and been classed as an important timber crop. However, its lack of genetic diversity (the species had a 14.5% genetic variation, from samples studied) resulted in its presence across its native range being drastically impacted by Cryphonectria parasitica (chestnut blight), simply because there was little inherent genetic variation that would have enabled some individuals to possess high levels of resistance. It’s a bit like if I cloned myself 3,000,000 times – there would be a lot of me, and I may provide a lot of good services, but if a killer virus comes along that I am poor at fighting against, all 3,000,000 of me are at huge risk of mortality. Put the same killer virus in a group of 3,000,000 different individuals however, and it is far more likely that not everyone will be at risk.
This is why threatened species with declining or already very small populations can suffer as a result of a limited gene pool. This limited gene pool, even if the species can regain its foothold, means that it may be susceptible for many generations (particularly if the species cannot breed with an individual with different genes outside of that gene pool – look up ‘genetic drift’ if you are interested in this). And for species with small populations that have a very small gene pool, there is huge risk of one ‘event’ wiping out the population – locally, regionally, or even nationally. That is also why, when collecting seeds for seed banks, seeds are sourced from a wide range of locations that would harbour different genotypes. It is of little use collecting 100 seeds from the same tree, as they are genetically highly similar (if not identical).
It may also make one consider whether using clonal propagation, even in urban areas, is a good idea. London’s recent i-Tree report suggests 21% of the 8,421,000 trees within Greater London are clonal – what does this mean in terms of urban forest resilience?
Source: Cooper, F. (2006) The Black Poplar – Ecology, History, and Conservation. UK: Windgather Press.
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